In electrophotographic printing, a photoconductive surface is charged, and is then exposed to image data to selectively discharge portions of the charged photoconductive surface. This forms a latent electrostatic image on the photoconductive surface. Charged toner material is then applied to the latent-image-bearing portion of the photoconductive surface to convert the latent electrostatic image into a developed image.
In image-on-image electrophotographic printing systems, this process is repeated a number of times to build a multi-layer image. Typically, each layer of the multi-layer image is one color separation. Together, these separations form a developed color image comprised of toner. This developed, or toner, image is then transferred, either directly, or indirectly via a transfer member, to a sheet of recording material. The developed, or toner, image is then at least semi-permanently fixed to the sheet of recording material. An example of this process is more fully described in U.S. Pat. No. 2,297,691.
In the image-on-image technique, the photoconductive member passes through the first charge/expose/develop station. A toned image is created on the photoconductive surface in a color corresponding to the color of toner contained in the first station. The image bearing member, containing this first toned image, then moves to a second charge/expose/develop station. The latent image for the second separation is created by exposing the photoconductor through the toned image from the first separation. Subsequent latent images are exposed through the image or images formed prior, on the same portion of the photoconductive surface, and then developed.
Different color features of an input image are formed at separate stations of the image forming device. Each station typically contains a charging substation, an exposing substation and a developing station. These stations and substations are arranged around, and can be strategically spaced relative to, the photoconductive surface. Thus, in such image forming devices, the photoconductive surface may be a photoconductive belt. The speed that the belt moves past these different stations can be strategically set to allow adequate time for: 1) uniform charging of the photoconductive surface, 2) sufficient exposing of the latent image and 3) sufficient developing of the image.
Commercial demands require reliable, high-speed production of quality images. Most image forming devices are capable of printing about 40-80 pages per minute. More sophisticated image forming devices can print up to 100 pages per minute or more. An example of such devices is described in U.S. Pat. No. 6,671,479.